Very, very easy to get to Old Town, and a huge free parking lot there as soon as you enter.
They do still have a free shuttle to zoo and sea world, but we did not get to use it. We didn't know until 9AM, that it left at 8:30 AM and came back at 11:30 AM. We had a rental car, and it was extremely easy to drive to.
There is really nothing within walking distance to see or do. They had a restaurant on site called the Pam Pam, but we didn't eat there. Heard from others it was greasy and over-priced. Saw several people having pizza delivered. Honestly, though, if you have a rental car, it is sooo easy to get to any of the main parts of San Diego.